It is effective to elevate a combustion temperature for improving efficiency of a steam turbine and a gas turbine for power generation.
At the present time, a steam temperature of a mainstream coal-fired power plant is 550 to 600° C. A ferritic heat-resistant steel is in use as a material for a turbine or a boiler. Since the ferritic heat-resistant steel is excellent in large steel ingot manufacturability, a large wrought product exceeding 10 tons is produced and utilized in a turbine rotor shaft and a boiler piping. However, since a durable temperature of the ferritic heat-resistant steel is at most about 650° C., the ferritic heat-resistant steel can not be used at a temperature higher than about 650° C. because of insufficient high-temperature mechanical strength.
In a gas turbine, a high temperature part uses a nickel base alloy having excellent high-temperature mechanical strength.
The nickel base alloy contains a solid solution strengthening element much, such as W, Mo or Co, and a precipitation strengthening element, such as Al, Ti, Nb or Ta, and has excellent high-temperature mechanical strength. A γ′ phase (Ni3Al), which is a main precipitation strengthening phase, has a property that the mechanical strength increases as a temperature increases and is very effective in improving the mechanical strength characteristics at a high temperature. When an element, such as Ti, Nb or Ta, is added, the γ′ phase is stabilized and can persist up to a higher temperature. Accordingly, when the nickel base alloy is to be improved in performance, it has been a main point of development how to stabilize the γ′ phase.
However, as the mechanical strength increases, hot forging becomes more difficult. Thus, it becomes impossible to produce, by forging, a rotor vane which bears largest load in the turbine or engine. Accordingly, the rotor vane is produced generally by precision casting (for example, see JP-A-09-272933). In the precision casting, since a workable weight is limited, a large part like a steam turbine rotor is difficult to be produced from a conventional high mechanical strength nickel base alloy.
On the other hand, JP-A-2009-097052 discloses a nickel base alloy having an excellent hot forging property and high-temperature mechanical strength in combination, which can be obtained by selecting an alloy element. The nickel base alloy can be preferably applied to a material of a steam turbine and a gas turbine.
As a factor inhibiting a nickel base alloy from becoming a large ingot other than the hot forging property, it is poor in large steel ingot manufacturability.
As is mentioned above, a nickel base alloy is added with many strengthening elements, and these elements are prone to segregate at the time of solidification. When segregation occurs in a steel ingot, cracks generate during hot forging, and a material becomes inhomogeneous so that necessary mechanical strength can not be obtained. Accordingly, an adequate material can not be obtained. As a size of a steel ingot increases, a cooling speed and a solidifying speed become slow and it results in a condition where segregation tends to generate.
With a conventional nickel base alloy, it is difficult to produce a large wrought material exceeding 10 tons as used in a steam turbine. Although there is a method where small parts are joined by welding to produce a large part, there is concern for a welding cost and a problem of reliability of the weld portions. Accordingly, a nickel base alloy that is unlikely to generate segregation and excellent in large steel ingot manufacturability is desired.